1. Field of Invention
The present invention relates to seismic measuring equipment. More particularly, the invention relates to a method for precisely adjusting the sensitivity of a polarized crystal hydrophone.
2. Description of Related Art
Due to the increasing difficulty and cost of finding petroleum resources in the world today, exploration techniques are becoming more and more technologically sophisticated. For example, many have found crystal hydrophones to be useful in petroleum exploration. Basically, hydrophones are used to measure seismic waves created by a source such as an air gun or a dynamite charge, to obtain detailed information about various sub-surface strata of earth.
As shown in FIG. 1, a typical crystal hydrophone 100 includes a diaphragm 102, a crystal 104, and a housing 106 that is typically filled with a gas 107. The diaphragm 102, which has front and rear sides 102a, 102b, is made from a material such as Kovar.TM. or a Berylium Copper compound, and is electrically connected to the crystal by a conductive epoxy 108. The crystal 104 is made from a material such as Lead Zirconium Titanate, and is silver-plated on its top 104a and bottom 104b to achieve better conductivity. The crystal 104 is initially polarized by applying a high-voltage electrical charge to the crystal 104. When the polarized crystal 104 experiences pressure resulting from a physical input, such as sound, fluid pressure, or another type of pressure, it produces a voltage representative of the pressure experienced. The crystal 104 is electrically connected to electrical output leads 110, 112. To protect the crystal 104 from contaminants, and to maintain the crystal 104 in atmospheric pressure, the crystal 104 and the rear side 102b of the diaphragm 102 are sealed within the gas-filled housing 106. The housing 106 protects the crystal 104 and diaphragm 102, and facilitates mounting of the hydrophone 100.
The diaphragm 102 functions to vibrate in response to physical pressures it experiences. The physical deflection of the diaphragm 102 is transferred by the epoxy 108 to the crystal 104, which deforms the electron structure of the crystal 104, causing an electrical potential to be provided across the leads 110, 112.
As mentioned above, hydrophones are often used in petroleum exploration in conjunction with seismic equipment. In one example of such an application (FIG. 2), a streamer 200 and a seismic source 202 are towed behind a ship 204. The streamer 200 is made up of many adjacent hydrophone "arrays" (not shown), where each array includes a plurality of hydrophones. All hydrophones of a single array are positioned on the same axis, so that all hydrophones respond similarly to a given pressure input. An exemplary hydrophone array may contain 16 hydrophones. The seismic source 202 is typically an air gun, a dynamite charge, or the like.
During operation of the system of FIG. 2, the seismic source 202 produces a large explosion. Seismic waves from the explosion travel through water 206, are refracted through various layers of earth 208, and are reflected back to the streamer 200, which senses the disturbances that it experiences and conveys them to the ship, where they are recorded by recording equipment 210. By studying these records, analysts can determine the makeup of the earth 208.
One analysis technique, called "amplitude versus offset," is based upon the fact that seismic waves have a different reflection coefficient for layered, non-isotropic rock at shallow angles than for homogeneous consolidated material. This is illustrated in FIG. 3, wherein the reflection coefficient (R.sub.c) versus angle of entry (.THETA.) curves are compared for a non-isotropic rock formation (curve 312) and a homogeneous formation (curve 314). The curves 312, 314 can be readily distinguished since, at a 45.degree. angle of incidence, the reflection coefficients of the curves 312, 314 vary by 12-15%. Accordingly, for a seismic wave, such as the wave 212, a wave reflected from a formation will probably be detected by the hydrophones near a region 214 of the streamer; with similar amplitudes for homogeneous and non-isotropic formations, while a seismic wave 215 detected by the hydrophones near a region 216 will have different amplitudes dependent on the formation lithology. In a manner that is beyond the scope of this disclosure, but well-known to ordinary skilled seismic geologists, a mathematical equation can be generated to represent the magnitude of the reflected signals received by the hydrophones of the streamer 200 as a function of their position in the streamer 200, the composition of the earth 208, the depth of the water 208, and other relevant factors. However, for such an equation to be useful and accurate, the sensitivities of the hydrophones must be uniform along the entire length of the streamer 200.
Having hydrophones of uniform sensitivity is also critical in calibrating the hydrophone arrays. In land-based operations, a geophone array may be easily calibrated with computer analysis data, since the positions of the seismic source and the array are fixed with respect to the earth. However, calibrating a hydrophone streamer 200 is more difficult since both the source 202 and the streamer 200 are moving simultaneously with respect to the earth. Therefore, if the hydrophones of a streamer 200 vary in sensitivity, the hydrophones cannot be computer-calibrated as effectively as desired, and data from the hydrophones is likely to contain errors
There are a number of techniques currently being used to help ensure hydrophone homogeneity. One method is to simply select or purchase hydrophones having a desired sensitivity. Usually, hydrophone manufacturers test their hydrophones to determine their sensitivities, and then sort the hydrophones into groups of similar sensitivities. Specifically, a nominal or average sensitivity is defined in volts output per unit of pressure input, and the hydrophones are sorted by their variance from the nominal value, e.g. +10%, +5%, -5%, -10%, etc. In a typical hydrophone batch, there is usually a maximum variation in sensitivity of .+-.20% from the average value. Therefore, the hydrophone purchaser may elect to purchase hydrophones of a single level, or purchase a batch of hydrophones and discard hydrophones that do not display the desired sensitivity. Discarding non-conforming hydrophones results in a great deal of waste, and is grossly cost inefficient.
Therefore, many use a second approach to obtain hydrophones of uniform sensitivity. This method uses a circuit 400 employing a capacitive divider (FIG. 4). Under this approach, a hydrophone array 402 is modeled as a power source 408, since the array 402 produces voltage in response to pressure, and an array capacitor 410, to represent the capacitance of the array 402. Under this approach, the output of a hydrophone 402 is electrically connected to a matching capacitor 404. The output across the matching capacitor 404 and hydrophone 402 is measured at a pair of terminals 412, 414 by signal-measuring equipment 405. The equipment 405 is modeled as a load capacitor 406. Since the capacitance 406 of the equipment is fixed, the voltage output at the terminals 412, 414 can be adjusted by changing the capacitance of the matching capacitor 404. Accordingly, the effective pressure sensitivity of the circuit 400 may be designated by either selecting an appropriate matching capacitor, or by utilizing an adjustable matching capacitor.
Although this method is useful for some applications, it is limited from the standpoint of accuracy. In particular, even when this method is used to normalize the outputs of multiple hydrophone arrays, the outputs of the arrays will still diverge by about .+-.5%, due to variations in the capacitances of the matching capacitors 404. This amount of inaccuracy is sometimes unacceptable, since some applications require accuracy of .+-.2% or .+-.1%. Even if more accurate capacitors can be found, this method is still limited from the standpoint of labor and cost, since a typical streamer 200 contains thousands of hydrophones, each of which may require a different capacitive adjustment. Moreover, the use of adjustable capacitors may be undesirable, since adjustable capacitors can introduce unwanted noise into the circuit.